1,3-Propanediol is used as a monomer unit for polyesters and polyurethanes and as a starting material for synthesizing cyclic compounds.
Various processes are known for the production of 1,3-propanediol which start either from C.sub.2 and C.sub.1 structural units or from a C.sub.3 structural unit, such as, for example, acrolein. When acrolein is used, this compound is first hydrated in the presence of an acidic catalyst, wherein 3-hydroxypropionaldehyde (3-hydroxypropanal) is formed. Once the unreacted acrolein has been separated, the aqueous reaction mixture formed during hydration still contains, in addition to 85% 3-hydroxypropionaldehyde, approximately 8% oxaheptanediol and further organic components in smaller proportions by weight. This reaction mixture is hydrogenated in the presence of hydrogenation catalysts to produce 1,3-propanediol.
According to Hatch et al. U.S. Pat. No. 2,434,110, catalysts suitable for hydrogenating 3-hydroxypropionaldehyde are those containing one or more metals having a hydrogenating action, such as, for example, Fe, Co, Cu, Ag, Mo, V, Zr, Ti, Th, and Ta. Raney nickel and Adkins' copper/chromium oxide may also be used as catalysts.
According to Arntz et al. DE-PS 39 26 136, the catalyst may be used either in suspended form or supported or as a constituent of fixed bed catalysts; homogeneous catalysts may also be used. Suspended catalysts which are mentioned are Raney nickel, which may be doped with various other catalytically active metals, and plantinum on activated carbon.
Prior art catalytic hydrogenation entails the risk that small quantities of the catalytically active element will be discharged into the product stream in the form of soluble compounds, making necessary further steps to separate the resultant contaminants. This may in particular be observed with suspended catalysts, such as, for example, Raney nickel.
Hydrogenation processes may be characterized by the conversions, selectivities, and space-time yields achievable therewith. Conversion indicates the number of moles of educt (in this case 3-hydroxypropionaldehyde) that are converted into other substances by hydrogenation. Conversion is usually stated as a percentage of the introduced moles of educt: ##EQU1##
In contrast, selectivity of the hydrogenation process is a measure of the number of moles of converted educt which are converted into the desired product: ##EQU2##
The space-time yield is another important characteristic for continuous hydrogenation processes, stating the achievable quantity of product per unit time and reaction volume.
When hydrogenating 3-hydroxypropionaldehyde to 1,3-propanediol on a large industrial scale, it is vital, with regard to the economic viability of the hydrogenation process and the quality of the product, for conversion and selectivity to be as close as possible to 100%. The 1,3-propanediol may be separated from the water as well as remaining 3-hydroxypropionaldehyde and secondary products contained in the product stream by distillation after the hydrogenation. However, this distillative separation is rendered very difficult by residual 3-hydroxypropionaldehyde and secondary products and may even become impossible due to reactions between the residual 3-hydroxypropionaldehyde and 1,3-propanediol to yield acetals, which have a boiling point close to the boiling point of 1,3-propanediol. Thus, the lower the conversion and selectivity, the poorer the achievable product quality.
In order to produce 1,3-propanediol economically, it is also important for the catalyst to exhibit high activity for the hydrogenation of 3-hydroxypropionaldehyde. The objective should thus be to find a process in which the smallest possible quantity of catalyst is necessary for the production of 1,3-propanediol; i.e., it should be possible to achieve the greatest possible conversion of 3-hydroxypropionaldehyde to 1,3-propanediol with a small volume of catalyst.
Conversion, selectivity, and space-time yield are influenced by the characteristics of the catalyst and by the hydrogenation conditions, such as reaction temperature, hydrogen pressure and duration of hydrogenation or, in the case of continuous hydrogenation, by the liquid hourly space velocity (LHSV).
When hydrogenating 3-hydroxypropionaldehyde to 1,3-propanediol, it should be noted that the main reaction is linearly dependent upon hydrogen pressure and time (space velocity in continuous processes), while reaction temperature has scarcely any influence. In contrast, the formation of secondary products is exponentially dependent upon temperature. Under otherwise identical conditions, secondary product formation may be observed to double per 10.degree. C., which correspondingly reduces the selectivity. Increasing the hydrogen pressure, in contrast, has a positive effect on selectivity, although the positive effect of pressure on selectivity is less pronounced than the negative effect of an increase in temperature, as hydrogen pressure increases the rate of the main reaction only linearly, while an increase in temperature increases the rate of the secondary reactions exponentially.
One important quality criterion for the catalysts used in the hydrogenation process is their operational service life. Good catalysts should ensure constant conversion and selectivity in the hydrogenation of 3-hydroxypropionaldehyde to 1,3-propanediol over the course of their service life. Known prior art hydrogenation processes, in particular those based on nickel catalysts, exhibit inadequate long-term stability in this connection. This entails more frequent replacement of the entire catalyst packing, which is associated with known problems in the disposal and working up of compounds containing nickel.
It is known from the 1991 Engelhard brochure Exceptional Technologies to hydrogenate aliphatic carbonyl compounds to the corresponding alcohols in the presence of ruthenium on aluminum oxide (Escalit).
It is known from the Degussa brochure Powder Precious Metal Catalysts (published 6/95) to hydrogenate aliphatic aldehydes to alcohols in the presence of supported ruthenium catalysts. Aluminum oxide is stated as the support in this case.
Arntz et al. European Patent EP-B 535 565 discloses a process for the production of 1,3-propanediol by heterogeneously catalyzed hydrogenation of 3-hydroxypropionaldehyde in an aqueous solution, in which the supported catalyst consists of titanium dioxide on which finely divided platinum is present in a quantity of 0.1 to 5 wt. % relative to the support. This process has the disadvantage that a relatively high hydrogenation pressure is required to provide substantially constant and high conversion over the service life of the catalyst. Moreover, due to its low activity, a relatively large quantity of platinum catalyst is required in order to achieve a sufficiently high level of conversion. Due to the high price of platinum, this correspondingly substantially increases the costs of the hydrogenation process.